One more difference.
The 898c HD SI Combo comes with the XHS 9 HDSI 180 T transdcuer and the Default setting is Hi-def Sidescan
The 899ci HD SI Combo comes with the XNT 9 SI 180 T transducer and the Default setting is Compact Sidescan.
The 899 can use the HDSI transducer at an additional cost.
Bottom line on the 899 is it is a wide screen version of the 798ci HD SI Combo out of the box, but can be upgraded to match the performance of the 898.